A plastic lens is lighter than an inorganic lens and is hard to break, and it is possible to perform dyeing thereon. Therefore, in recent years, the plastic lens has been rapidly widespread as a candidate material forming a spectacle lens, a camera lens, and the like.
In addition, higher performance has been required for a resin for a plastic lens, and high refractive index, high Abbe number, low specific gravity, and high heat resistance have been required. Various resin materials for a lens have been developed and used so far.
Among them, technological development on a polyurethane resin has been actively performed, and the present inventors have also performed various kinds of technological development on a plastic lens in which this polyurethane resin is used (for example, refer to Patent Document 1, Patent Document 2, and Patent Document 3).
An example of a representative resin among them is a resin obtained by reacting a pentaerythritol mercaptocarboxylic acid ester with a polyiso(thio)cyanate compound. This resin is colorless and transparent, and has characteristics of high refractive index and low dispersion. This resin is one of the most suitable resins for a plastic lens having excellent impact resistance, dyeability, workability, and the like.
The pentaerythritol mercaptocarboxylic acid ester is produced through a so-called direct esterification method that a ordinal polyhydric alcohol is reacted with a ordinal mercaptocarboxylic acid in the presence of an esterification catalyst, while distilling off water, which is produced as a by-product outside a system (refer to Patent Document 4).
Pentaerythritol which is a raw material of this ester compound is usually produced by condensing acetaldehyde and formaldehyde. The purity of pentaerythritol is usually about 90 wt % and pentaerythritol contains several kinds of impurities. Among them, an example thereof includes bispentaerythritol which is a bi-molecular condensate of formaldehyde of pentaerythritol. If the content of this bispentaerythritol is greater than 5 wt %, it is known that there is a possibility that problems such as difficulty in performing release from a mold after completion of polymerization with a polyisocyanate compound or generation of bubbles in the obtained lens may be caused (refer to Patent Documents 5 and 6).
In addition, it is known that a pentaerythritol mercaptocarboxylic acid ester can be obtained which does not become cloudy even when reacted with poly(iso)thiocyanate, by controlling the content of alkali metal and alkaline earth metal to be a specific amount or less with respect to pentaerythritol used as a raw material (refer to Patent Document 7).